1. Field of the Invention
This invention relates to a permanent magnet type stepping motor, and in particular to an improvement to a permanent magnet type stepping motor provided with a stator which has coils wound onto a plurality of magnetic poles that project radially inwardly and are spaced around the inner periphery of a cylindrical yoke, with pole teeth formed on the radially inner tips of the magnetic poles, and a rotor comprising a disc-like permanent magnet coaxially sandwiched between two substantially cylindrical rotor pole pieces with pole teeth around their circumferences that radially face the pole teeth of the stator across a gap.
2. Description of the Prior Art
FIG. 1(a) is a longitudinal section through a conventional stepping motor, and FIG. 1(b) is a view thereof in transverse section. In the drawings, numeral 1 denotes a stator core provided with a plurality of magnetic poles that project radially inwardly from the inner periphery of a cylindrical yoke, 2 denotes a stator coil bobbin surrounding each magnetic pole, 3 denotes a stator coil wound onto the bobbin 2, 4 denotes circumferentially spaced pole teeth on the radially inner tips of the stator core 1, and 9 denotes pole teeth of a rotor.
In FIGS. 1(a) and 1(b), there are eight each of the magnetic poles and stator coils 3; the stator coils 3, which are positioned symmetrically around a rotor shaft 7, are connected in series to produce magnetic poles of the same polarity, so that four coil groups are formed as a result.
End brackets 5 are held in contact with the front and back of the stator core, the rotor shaft 7 is borne rotatably in bearings 6 in the end brackets 5, and the rotor comprising rotor pole pieces 8 and a permanent magnet 10 is attached to the rotor shaft 7. The arrangement is such that the pole teeth 4 provided on the stator magnet poles and the circumferentially spaced pole teeth 9 provided on the rotor pole pieces 8 are at the same pitch, and the pole teeth 9 on one rotor pole piece are offset circumferentially by 1/2 a pitch from those of the other rotor pole piece 8.
In the stepping motor shown in FIGS. 1(a) and 1(b), the pole teeth 9 of the rotor are attracted so as to line up with the pole teeth 4 of the magnetic poles on which conducting coils are wound by current pulses conducted sequentially to the several stator coil groups wound on the stator core, four coil groups in this illustration, and the rotor moves by 1/4 of the pitch of the pole teeth 9 at every change in the coil group to which a current pulse is conducted.
Thus the number of changes in the conduction of current pulses to the coils in the stepping motor is proportional to the rotational angle of the rotor; therefore a control unit can be simplified, and this motor has been applied extensively of late to various applications needing control units.
However, in some kinds of control unit it is necessary to make the complete unit very thin axially and to incorporate several of these units in a given sized space, therefore the axial dimension of the stepping motor used in this unit must be minimized, so that in the stepping motor shown in FIGS. 1(a) and 1(b) the axial length L is made small and the diameter large, to solve this problem.
In the motor of FIGS. 1(a) and 1(b), the factors governing the length L are the thickness of the stator core 1, the thickness of the stator coil 3 and of the bobbins 2 onto which the coils 3 are wound, the thickness of the end brackets 5, and the spaces necessary for insulation between each of the end brackets 5 and the stator coils 3. In this motor, the thickness of the end brackets 5 and the spaces necessary for insulation between the end brackets 5 and the coils 3 are almost constant regardless of the size of the motor, but the thicknesses of the stator core 1 and of the stator coils 3 are closely related to the output of the stepping motor. When the ratio of the size to thickness exceeds a certain limit, the output drops suddenly to present a problem. In the constitution of FIGS. 1(a) and 1(b), the factors determining the output of the stepping motor are the quantity of changing magnetic flux acting between the pole teeth 4 of the stator and the opposing pole teeth 9 of the rotor, the number of turns of the stator coils 3, and the current flowing therethrough. The thickness of the stator core 1 is nearly the same as the sum of the thicknesses a, c of the rotor pole pieces 8 plus the thickness b of the permanent magnet 10, i.e., a+b+c; however the only part of the core thickness influencing the output is the part thereof, corresponding to the thicknesses a and c, that faces the rotor poles 8; where-as the part of the core corresponding to the thickness b of the permanent magnet 10 does not face a rotor pole, and hence does not contribute to the torque generation of the rotor but only increases the thickness of the core unnecessarily, and consequently reduces the coil space so that the output can not be increased.